The present invention relates to an indicator system which is responsive at a temperature-dependent rate to yield a visually-distinct indication of cumulative thermal exposure of an associated product in excess of a predetermined time-temperature integral. More particularly, the invention relates to such a system which is attached to a unit of thermally-sensitive perishable product and is activated at the time of introduction of the unit into commerce to initiate the monitoring of the unit""s cumulative exposure to harmful temperatures. The system provides for the generation of a visually-distinct color which signals the end-point of the safe, useful shelflife of the product when the allowable time-temperature integral is exceeded. In a preferred embodiment, the indicator system of the invention comprises the use of a direct thermal label paper and an additional element which is applied to the label to react therewith and initiate a time- and temperature-dependent color change useful in monitoring the cumulative ambient temperature exposure of a labeled perishable product.
Color-forming or color-changing temperature-sensitive indicators which are capable of monitoring the handling of perishable goods are generally known and their use for this purpose is increasing. The utility of such indicators is to signal when a perishable article to which the indicator is attached has reached the point of quality loss, or unsafe condition, due to periods of excessive temperature exposures after which the product should no longer be used, or the product should be closely scrutinized to ensure suitable quality prior to being used. Indicator systems of this nature are important to ensure the quality and safety of perishable foods, pharmaceuticals, chemicals, and other such sensitive items.
In the case of perishable foods, modern packaging technologies are employed in an attempt to extend the shelf life of such products as meat, poultry, and fish. For example, plastic films with various gas permeabilities and barrier properties are used to maintain gas mixtures in the package to reduce the proliferation of aerobic spoilage organisms. However, the microbiology of food contained in this type of package permits anaerobic pathogens to grow in this modified gas atmosphere under suitable temperature conditions with no organoleptic sign of spoilage. A food product thus exposed to such temperature abuse can produce a lethal concentration of pathogens, yet provide no discernable sign of spoilage, a condition which can lead to serious health consequences if the product is consumed. It is therefore a safety feature for the consumer to have an indicator system associated with the perishable product in order to provide some visual indication that can warn of exposure of the product beyond an acceptable combination of time and temperature, i.e. a critical xe2x80x9ctime-temperature integralxe2x80x9d.
With respect to highly perishable foodstuffs, e.g., meat, poultry, and fish, it is advantageous for the indicator system to be placed on each unit of sale so that continuous monitoring is carried out from the time of packing until the time of use. Otherwise, there would be no indication that a packaged product has been subjected to an unknown temperature excursion, such as could result during transportation and distribution or from intermittent customer handling or removal from a refrigerated case, which could lead to significant quality degradation and health endangerment.
In order to be most effective, indicator systems should be formulated and employed to provide a visual indication, such as a change in color, contemporaneously with the generation of conditions of spoilage in the associated perishable products. To this end indicator systems should ideally have a rate of visual change paralleling the deterioration rate of the associated product. Although such an ideal performance is not readily achievable in view of the myriad conditions influencing product spoilage, indicator systems should at least be operable only during their association with a product. That is to say, an effective indicator system should not be susceptible of responding to or registering temperature gradients to which it has itself been exposed between the time of its own manufacture and the time it is ultimately attached to the intended perishable product. Only in this manner is an indicator system able to reliably monitor the complete thermal history of an associated perishable product throughout the various phases of that product""s storage and distribution.
In earlier attempts to meet the need for independent response in time-temperature indicator systems, such as freeze/thaw monitors and shelflife markers, the indicator product, in the form of label, tag device, or the like, was isolated from actinic temperatures immediately upon manufacture, such as by freezing or at least cooling to a non-active temperature. For example, widely-used labels comprising diacetylene monomer inks registering irreversible color generation upon polymerization as a function of ambient temperature were required to be stored from the time of manufacture at temperatures below the threshold of significant color-forming polymerization. Such expedients were effective as long as storage and handling conditions were scrupulously monitored; however, these indicator system products were themselves subject to the same vagaries of human behavior as were the perishable products they were intended to protect, and the desired reliability could be compromised. The expenditure, albeit occasionally futile, of equipment and resources in an attempt to ensure the required storage conditions sometimes outweighed the initial cost of the indicator product.
Thus, apart from the fundamental requirements of reproducibility and economical production, an acceptable indicator system must be isolated in an economical and xe2x80x9cfail-safexe2x80x9d manner from conditions which would otherwise initiate its own temperature response reaction and detract from its ability to register a true time-temperature integral vis-a-vis a given perishable product with which it is ultimately to be associated. The most reliable such means for isolation is to formulate or configure the indicator system to be in an inactive state which is made active only at the time of its association with the product unit to be monitored. The present invention provides such an economical, reliable, and activatable time-temperature indicator system.
A number of activatable time-temperature indicator systems have previously been proposed, yet none provides a ready and economical means for preventing premature initiation of the underlying temperature-sensitive indicator reaction. In a majority of such systems which comprise, for example, a label which is to be affixed to a perishable product, potential co-reactant components, such as precursors to an ambient temperature color-forming reaction, are located in close proximity, as in contiguous layers or interspersed mixtures, yet are maintained in reactive isolation by means of additional intervening layers, encapsulating films, or the like. Each of such isolating means, however, introduces additional expenses of resources and manufacturing operations. Further, these indicator products continue, due to the close proximity of potential reactants, to be susceptible to inadvertent, premature activation, such as where pressure-rupturable isolating encapsulations are subjected to mishandling or dormant light-sensitive co-reactants experience vagrant actinic exposure, or where other acceptable storage conditions are exceeded.
Another consideration which has contributed to the limited acceptability of currently-available activatable indicator systems is the excessive economic expenditures in materials and manufacturing operations resulting from the requirement for indicator and activating compositions or means to be individually formulated and assembled, along with isolation means, into the final composite indicator system product. Thus, from the viewpoint of economical acceptability alone, it is desirable to make available to the industry a time-temperature indicator product which comprises the use of a minimum of effective and low-cost components and resources.
To this end, the present invention utilizes as a visually-responsive co-reactant element a readily-available common or commodity material, such as a heat-printable label product which comprises a composition, e.g., a mixture of potentially-reactive color-forming precursors, having inherent resistance to activation under less than extraordinary ambient conditions of temperature or the like. The invention further comprises an additional, economical activator product element to be combined with the visually-responsive label product at the time of ultimate application to a perishable product to be monitored. In this manner the invention not only eliminates potential dangers of premature indicator activation, it also reduces significantly the expense of a dedicated time-temperature indicator system by incorporating as a major component existing, readily-available general-purpose, visually-responsive products, such as temperature-responsive thermographic commodity labels.
Thermally-responsive marking or printing products, particularly direct thermal label products, are currently in wide distribution and use. Due to their simple response to heated marking indicia, these clean, non-smearing products are instrumental in the rapid replacement of marking or printing equipment requiring replenishment of inks or colored printing ribbons. Thus, thermal printing devices are incorporated to a nearly universal degree in point-of-sale labeling equipment, such as weighing scales and other dispensing devices employed at locations marketing perishable foods and other commodities. It is in conjunction with thermographic label products used at such locations that the time-temperature indicator activator element product embodiments of the present invention find particular application.
The present invention comprises a time-temperature indicating system for monitoring the elapse of a combination of time and ambient temperature. Such a system is particularly useful in monitoring the exposure of a perishable foodstuff or commodity product to a critical integral of such time and temperature which would result in spoilage or unacceptable degradation of the quality of the perishable product.
In a preferred embodiment, the invention takes the form of a label having a paper or film substrate bearing a coating comprising a first, or primary, composition which is thermally-responsive to yield a visually-apparent change, such as development or intensification of a color. This embodiment further comprises a second composition, either self-formed or borne on a second substrate, which is combined in contact with the label coating to activate or facilitate an ambient thermal response of the primary composition. From the moment of such activation, typically effected at the moment of association of the label, by self-adhesion or other common means, with a unit of perishable product, the thermally-responsive composition of the combination system proceeds inexorably in its color-forming reaction, at a varying rate influenced by ambient temperature, toward the critical integral of time and temperature which, by a predetermined level of developed color density, will signal the end of useful life of the associated perishable product.
The primary composition label may preferably be a self-adhesive thermographic label commonly dispensed from weighing station equipment at supermarkets, delicatessens, fast-meal emporia, meat, fish, and poultry processing plants, and the like and which generally takes the form of a xe2x80x9cdirect thermal paperxe2x80x9d comprising a primary color-forming high-temperature printing composition. Other products incorporating thermographic compositions, such as facsimile and thermal-imaging papers and films, represent additional sources of first composition element components useful in the present invention.
The second, activating element component preferably takes the form of a self-adhesive tab or label comprising a substrate and an adhesive composition which additionally comprises an activating component that, e.g., when applied to the direct thermal label coating, combines with the primary composition to enable its color-forming reaction, for example, to proceed within a lower temperature range than that required for the designed thermographic response of the label product, the lower range being typically that which lies above the range of safe storage temperatures for the subject perishable product and within which product spoilage or quality deterioration is likely to occur. In variant embodiments, the activating co-reactant component may be further prevented from premature action upon the primary color-forming composition by isolation within pressure-rupturable capsules or may comprise a photolabile compound requiring exposure to actinic radiation. Alternatively, the activating composition may be applied directly, e.g., in fluid or laminate form, to the primary composition coating at the time of point-of-sale label marking and application to the subject product unit.
In yet another embodiment of the invention, the activating tab composition may comprise as the activating component a co-reactant which when combined with one or more of the reactant components of the primary high-temperature printing composition will form a secondary color-forming composition which proceeds at its own temperature-dependant reaction rate within a lower intermediate ambient temperature range that presents the danger of food or perishable spoilage. Where the intermediate range of spoilage temperature may vary within an enterprise location, e.g., as differing among fish, meat, and poultry within a supermarket environment, a variety of activating element tabs may be automatically made available to activate the primary label composition at different ambient temperature reaction ranges and at different critical time-temperature integrals.
Operator selection or weighing station intelligence would determine from perishable product identity the appropriate activator tab supply.
The visually-apparent endpoint of a critical time-temperature integral may take any convenient or intuitive form depending upon the anticipated arena within which its observance would be manifest. That is to say, the activator may be applied to the primary label composition in a distinct shape or design, such as would create or obscure a barcode to which in-market equipment would respond, or which would present a gradient-matching color density or colored warning message for the benefit of the purchasing consumer.
Direct thermal papers and other similar products comprising a primary high-temperature color-forming printing composition are widely known and commercially available from a number of industry sources. Each of this type of product, which finds ubiquitous use in facsimile machines, supermarket or food processor price/weight label machines, point-of-sale credit card registers, and the like, normally comprises a substrate coated with a mixture of co-reactant materials capable of forming a visually-distinct mark or color when exposed to elevated temperatures in excess of about 60xc2x0 C. Commonly, these sheet products comprise a substrate of paper, polymer film, foil, or the like coated with a composition of co-reactant compounds or materials which combine at a predetermined temperature to form, in a normally irreversible reaction, a color highly contrasting with the unreacted composition background.
Thermally-responsive color-forming co-reactant compositions are well known in the art. Typical of such compositions are pH-sensitive combinations comprising azo dye precursors, co-reactant coupler compounds, and alkaline reaction initiators, or colorless or pale leuco dyes and proton-donating substances, i.e., acids, which together form a contrasting dye color upon application of prescribed temperature. The color-forming response of such compositions to applied heat may result directly from the inherent thermodynamics of the color-forming reaction, or may be due to activation of a thermolabile co-reactant or heat-initiated release of a co-reactant from protective encasement within a fusible matrix or encapsulation.
Representative of the myriad color-forming compositions and dye, acid co-reactant, and binder matrix components which are known in the art and which, although not per se constituting an essential part of the present invention, are particularly useful in its practice are color precursors such as fluoran, lactone, phthalide, or triaryl methane dyes, e.g., crystal violet lactone, 3, N-cyclohexyl-xcex2-methyl-amino-6-methyl-7-anilino fluoran, or 3-pyrolidino-6-methyl-7-anilino fluoran, acid-producing co-reactants such as p-benzyl hydroxybenzoate, bisphenol A, phenolic condensation products, and low-melting organic acids or esters, and cellulosic and mono- and co-polymeric binder materials such as vinyl acetates, alcohols, or pyrrolidones and acrylates or acrylamides. Exhaustive lists of such known components and compositions may be found in the published art, for example, in patent literature by Iwata et al., U.S. Pat. No. 4,370,370; Glanz, U.S. Pat. No. 4,535,347; Arbree et al., U.S. Pat. No. 4,591,887; Kang, U.S. Pat. No. 4,898,849; Smith et al., U.S. Pat. No. 5,071,821; Kawakami et al., U.S. Pat. No. 5,288,688; and Hoffmann et al., U.S. Pat. No. 5,354,724; each of which is incorporated herein by reference.